1. Technical Field
The present invention relates to methods and apparatus for augmenting hearing and, more particularly, to improvements in hearing aids of the type wherein acoustic vibrations are transmitted to the ear via bone structure.
2. Discussion of the Prior Art
It is known in the prior art that certain hearing impaired persons experience significantly improved hearing function when provided with devices that operate to convert sound to mechanical vibrations that are applied to bones in the skull. These vibrations ultimately stimulate the cochlea, resulting in a perception of sound. For examples of such devices, reference is made to the discussion in U.S. Pat. No. 4,606,329 (Hough). See also, U.S. Pat. Nos. 4,612,915 (Hough et al) and 4,774,933 (Hough et al). These patents describe prior art bone conduction devices and are primarily concerned with prostheses that are surgically embedded into the mastoid or temporal bone. The express purpose of the technique is to have the prosthetic device integrate into bone tissue so that it may thereafter be used as a source of vibrations that ultimately stimulate the cochlea. Although the technique has merit, in practice it has a number of disadvantages. For example, all tissue-integrated prostheses must be selectively utilized in order to preclude an unacceptably high failure rate, thereby severely limiting the population for whom the technique is available. In addition, there is a real risk of infection developing from tissue-integrated prosthetic devices; in this regard, a middle ear infection can result in the occurrence of life-threatening mastoiditis or meningitis. Further, from a cosmetic perspective, the prosthetic device requires a very evident supercutaneously mounted transducer for converting sound waves to electrical audio signals. Finally, implantation of the prosthetic device involves invasive surgical procedures that must be performed by specially trained otolaryngologists.
In U.S. Pat. No. 2,161,169 (Jefferis) there is disclosed a bone conduction hearing aid mounted in a denture. The denture includes at least one artificial incisor in which a microphone is mounted to receive sound and convert it to an electrical audio signal. That signal is transmitted by wire to a transducer located at the rear of the denture for converting the audio signal to mechanical vibrations that are conducted to the ear through the bone structure of the jaw. The transducer is covered by artificial molars in the denture, as is a battery employed to power the unit. This arrangement has a major disadvantage in that it only has utility for persons lacking at least one natural incisor and at least three natural molars. Moreover, in order to impart vibrations to the bone structure, the built-in transducer is provided with a diaphragm-mounted lug to impart vibrations to a denture wall which, in turn, is required to impart these vibrations to bones in the jaw. The denture wall must, therefore, be configured to provide a tight fit on the patient's gums, with the result that discomfort is likely. Moreover, as the gums automatically change, the required tight fit is compromised with the result that the vibrations are ultimately not efficiently transmitted to the bones in the jaw.
Another approach to hearing augmentation is disclosed in a series of patents to Puharich et al, namely U.S. Pat. Nos. 2,995,633, 3,156,787, 3,170,993 and 3,267,931. The devices and techniques described in these patents utilize a microphone-transmitter worn by the subject and arranged to convert sound to an audio-modulated radio frequency signal that is transmitted to a radio receiver in the subject's mouth. The receiver, which recovers the audio modulation, must be located adjacent a vital (i.e., live) tooth, or under a cap or filling of that vital tooth. The recovered audio signal is electrically coupled to the Facial Nerve (i.e., cranial Nerve VII) or the Trigeminal Nerve (i.e., Cranial Nerve V) in the tooth itself. It is the electrical transmission of the audio signal to and through this specific nerve network that is said to result in augmented hearing. In this regard, Puharich et al expressly state that the sensation of hearing does not result if vibrations are electrically insulated from the nerves, and that bone conduction of vibrations, of itself, is of very limited value as an aid to hearing using their apparatus and technique. Among the problems associated with the Puharich et al approach is the fact that the requirement for electrical coupling of the audio signal to the nerve network renders the system anatomically and physiologically difficult to implant and apply voltage without causing pain. It is also subject to failure by short circuits from leakage of saliva, and the like, as the fillings will be loosened by the underlying crystals. In addition, the Puharich et al approach requires that at least two of the subject's teeth be crowned, capped or otherwise treated to house the system components. Finally, it has been my observation in direct contradiction to Puharich et al, that transmission of the electrical audio signal via the Facial Nerve network of the patient is not nearly as efficient as bone-transmission of mechanical vibrations, if the mechanical vibrations are applied in the proper direction with sufficient voltage.